[rust png] Reinforce `testonly`-ness of some users of `libpng`.

This CL extracts a separate `third_party/libpng:libpng_for_testonly`
target which:

1) Is a `group` that proxies/forwards to the `libpng` target by listing
   it as its `public_deps`
2) Has limited visibility (based on the new `libpng_testonly_visibility`
   introduced in the refactored `third_party/libpng/visibility.gni`).
   The new visibility is almost unchanged, except for explicitly
   spelling out some targets instead of using wildcards
3) Is marked as `testonly`

Bug: 443128323
Change-Id: I15626f7ece79304f9649057e7e215c3e72d15b92
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6976870
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1521521}